1. Field of the Invention
The present invention relates to a transmission device, and more particularly to a chainless transmission device for a bicycle.
2. Description of Related Art
With reference to FIG. 6, a conventional chainless transmission device for a bicycle in accordance with the prior art comprises a front casing (50), a rear casing (52), a crank axle (51) and a transmission shaft (54). The front casing (50) is securely attached to a bottom bracket (62) on the frame (60) of the bicycle. The crank axle (51) is rotatably received in the front casing (50). Each end of the crank axle (51) extends out from the front casing (50) to which a pedal (not shown) is attached. A drive gear (55) is attached to the crank axle (51) to co-rotate with the crank axle (51). A driven gear (58) is attached to the rear wheel axle (66) in the hub (64) of the rear wheel (not shown) and received in the rear casing (52). The hub (64) is held between the rear casing (52) and a chain stay (68) connected to the bottom bracket (62). The rear wheel of the bicycle is mounted on the hub (64). A tube (53) is connected between the front casing (50) and rear casing (52) to receive the transmission shaft (54). A front gear (56) is attached to one end of the transmission shaft (54) to engage with the drive gear (55). A rear gear (57) is attached to the other end of the transmission shaft (54) to engage with the driven gear (58).
Accordingly, when a rider rotates the pedals connected to the crank axle (51), the crank axle (51) will rotate relative to the front casing (50). The hub (64) and the rear wheel will rotate as a result the transmission of the rotational energy through the drive gear (51), the front gear (56), the transmission shaft (54), the rear gear (57) and the driven gear (58). The transmission device can replace the traditional chain transmission device of the bicycle.
However, when the rear wheel hits an object, the wheel with the hub (64) will move upward and vibrate. Because the rear gear (57) is securely attached to the transmission shaft (54) and engages with the driven gear (58), the rear gear (57) will move upward and vibrate with the hub (64). Consequently, the transmission shaft (54) may be bent causing the transmission shaft (54) to rotate along a bent axis. The friction increases when the transmission shaft (54) rotates, and the transmission efficiency of the transmission shaft (54) is reduced.
Since the conventional front casing (50) is received in the bottom bracket (62), a base (502) is formed on the front casing (50) so the tube (53) can be connected to the front casing (50). A notch (622) is defined in the bottom bracket (62) to engage with the base (502), such that the base (502) is exposed from the notch (622) to connect with the tube (53). However, because the bottom bracket (62) must be machined to fit the front casing (50), the process for manufacturing the bicycle frame (60) is complex. Furthermore, if the actual position of the notch (622) is not in the desired position due to an error in the forming process, it is extremely difficult to install the transmission device in the bicycle frame (60).
To overcome the shortcomings, the present invention tends to provide an improved chainless transmission device to mitigate or obviate the aforementioned problems.
The main objective of the invention is to provide an improved chainless transmission device that is easily installed in the bicycle frame. The transmission device has a front casing, a rear casing, a crank axle and a transmission assembly. The transmission assembly has a transmission shaft, a drive gear, a front gear, a rear gear and a driven gear. A tube is connected between the front casing and the rear casing to receive the transmission shaft. Two ears extend from the front casing. A connector is secured to each ear to connect to the bottom bracket of the bicycle. Consequently, the bottom bracket does not needed to be machined. The transmission device can be mounted directly on a conventional bicycle frame. To install the transmission device on the frame becomes easier.
Another objective of the invention is to provide an improved chainless transmission device wherein the transmission shaft can move slightly and rotate relative to the rear gear to prevent the transmission from being bent. The transmission efficiency and the useful life of the chainless transmission device are improved.
Other objectives, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.